Lipoproteins - Molecular Biochemistry II Lipid Digestion...

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Copyright © 1999-2008 by Joyce J. Diwan. All rights reserved. Molecular Biochemistry II
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Digestion & transport of lipids poses unique problems relating to the insolubility of lipids in water . Enzymes that act on lipids are soluble proteins or membrane proteins at the aqueous interface. Lipids, & products of their digestion, must be transported through aqueous compartments within the cell as well as in the blood & tissue spaces.
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Bile acids (bile salts) are polar derivatives of cholesterol, formed in liver and secreted into the gall bladder. They pass via the bile duct into the intestine, where they Bile acids are amphipathic , with detergent properties. OH CH 3 CH 3 C O O R 2 R 1 HO R 1 = OH or H R 2 = H or NH - CH 2 - COOH or NH - CH 2 - CH 2 - SO 3 - H Bile Acids
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Bile acids emulsify fat globules into smaller micelles, increasing the surface area accessible to lipid-hydrolyzing enzymes. They also help to solubilize lipid breakdown products (e.g., mono- & diacylglycerols from triacylglycerol hydrolysis). OH CH 3 CH 3 C O O R 2 R 1 HO R 1 = OH or H R 2 = H or NH - CH 2 - COOH or NH - CH 2 - CH 2 - SO 3 - H Bile Acids
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Secretion of bile salts & cholesterol into the bile by liver is the only mechanism by which cholesterol is excreted . Most cholesterol & bile acids are reabsorbed in the small intestine, returned to the liver via the portal vein, & may be re-secreted. This is the enterohepatic cycle . Agents that interrupt the enterohepatic cycle are used to treat high blood cholesterol. Examples: Synthetic resins, as well as soluble fiber (e.g., oat bran fiber and fruit pectin), that bind bile acids &/or cholesterol , prevent absorption/reabsorption. A recently introduced drug ezetimibe acts on cells lining the lumen of the small intestine to inhibit absorption of cholesterol.
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Pancreatic lipase (secreted into the intestine), catalyzes hydrolysis of triacylglycerols at positions 1 & 3, forming A protein colipase is required to aid binding of the enzyme at the lipid-water interface. Monoacylglycerols & fatty acids are absorbed by intestinal epithelial cells. Within intestinal epithelial cells triacylglycerols are resynthesized. H 2 C HC H 2 C O O O C R 1 O C C R 3 O R 2 O - O C R 3 O H 2 C HC H 2 C O O OH C R 1 O C R 2 O H 2 O triacylglycerol 1,2-diacylglycerol fatty acid +
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2 is secreted by the pancreas into the intestine. It hydrolyzes the ester linkage between the fatty acid & the hydroxyl on C2 of phospholipids. Lysophospholipids , the products of Phospholipase A 2 reactions, are powerful detergents . C
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Lipoproteins - Molecular Biochemistry II Lipid Digestion...

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